The present invention relates to a reactor system of a boiling water type atomic power plant, and in particular to a reactor system suitable for reducing an outflow of reactor coolant in the case of a break accident of a main steam piping and for reducing an amount of structural materials for a main steam piping system and a turbine building.
In a reactor system of a prior, typical boiling water type atomic power plant, steam generated in a reactor pressure vessel is supplied from a reactor building to a turbine building through a main steam piping. The main steam piping is disposed in a main steam tunnel between the reactor building and the turbine building, and inside and outside main steam isolation valves are provided near a primary containment vessel. Main steam having entered into the turbine building is supplied through a main steam stop valve and a main steam control valve to a main turbine to drive the same. The main steam stop valve acts to stop a supply of main steam to the main turbine in case of turbine trip, a period of time required for the closure of the stop valve being about 0.1 second such that the valve can be closed several ten times as fast as the main steam isolation valves. The inside and outside main steam isolation valves are opened during normal operation of the plant, but are closed in case of a break accident of the main steam piping to prevent the outflow of reactor coolant within a predetermined period of time. Accordingly, it is desired in case of a break accident of the main steam piping to quickly shut off all of the isolation valves for the reduction of an exposure dose. On the other hand, such quick closure of the isolation valves causes a severe transient phenomenon of a pressure increased in the reactor pressure vessel. More specifically, upon such quick closure of the isolation valves, pressure in a closed space, that is, a space in the reactor pressure vessel is rapidly raised by steam as generated, so that neutron flux or heat flux of the fuel assembly changes to severely affect the reactor system. To cope with the matter, a main steam stop valve of a quick closure type is provided in the turbine building to allow a space in the main steam tunnel to accommodate the generated steam, and a period of time for the closure of the inside and outside main steam isolation valves in case of accidents is limited to about 3 to 4.5 seconds taking into account of the volume of the reactor building and main steam tunnel. Consequently, some amount of reactor coolant in the form of vapor will flow outside of the reactor pressure vessel before the inside and outside main steam isolation valves are completely closed upon a break accident of the main steam piping.
An aseismic design condition for a reactor system is prescribed in "U.S. Regulatory guide 1.29 Seismic design classification" (Revision 3, September 1978), in which it is prescribed that an area on the side of a reactor pressure vessel from an outside main steam isolation valve and an area on the side of a main turbine from the outside main steam isolation valve are assorted into different seismic classes in design of a boiling water type atomic power plant. More specifically, a portion of the main steam piping on the side of the reactor pressure vessel from the outside main steam isolation valve must be designed in the highest seismic class: Seismic Category I while the main steam stop valve and a portion of the main steam piping on the side of the main steam turbine from the outside main steam isolation valve must be designed for the large seismic load in the seismic category I class on the basis of an evaluation of exposure dose which is assumed allowing for a period of time (3 to 4.5 seconds) required for the closure of the main steam isolation valves upon a break accident of the main steam piping. The turbine building, which is required to have a shield function, is designed in non-category I class, in which a seismic design load is small. However, since the main steam piping and the main steam stop valve in the turbine building are designed in the seismic category I class, the construction for supporting the main steam piping and the main steam stop valve must also be designed in the seismic category I class (seismic load is much larger than in the case of non-category I class) to have a large strength.
As described above, there is room for improving a design of a prior reactor system in safety and economical efficiency.
In a reactor system of a prior underground type atomic power plant, reactor building and turbine building are spaced away from each other by a large distance due to the terrain of the plant as compared with an on-ground type reactor system, so that a main steam piping is large in length. In this connection, Japanese Patent Laid-Open Publication No. 59-63596 describes a design for a reactor system, in which a quick closure valve is provided between an outside main steam isolation valve and a main steam stop valve on a main steam piping extending through a main steam tunnel. In this design, steam and water flowing out upon a break accident of the main steam piping are prevented from entering into a reactor building and a turbine building, and supporting structures downstream of the quick closure valve can be designed in the seismic class B (Japanese non seismic category I class) rather than in the seismic class B (Sl) equivalent to the class A (here, P(Si) and A are Japanese category I classes), so that an advantageous layout for pipings and buildings in terms of economical efficiency can be embodied. In case such arrangement is applied to a reactor system of a boiling water type atomic power plant on the ground, in which a main steam piping is small in length to have a small spatial volume, when a quick closure valve is quickly closed in a break accident of the main steam piping, a subsequent transient pressurization becomes too unstable and severe to be roped with an existent construction. Furthermore, it is not advisable in terms of plant thermal efficiency to make the main steam piping long in a boiling water type atomic power plant.